Nutrient Supply From Organic Soil Amendments: Implications For Fertility Management

Slow release nutrient amendments, particularly those from organic sources (i.e. plants and animals), are becoming increasingly popular in greenhouses and in certified organic farming. Depending on what material is being used, these amendments can vary in the amount, timing and balance of nutrients released. This makes managing soil fertility very challenging if you are a gardener, farmer or greenhouse operator.

We are testing four commercially available organic amendments: poultry manure compost, feather meal, alfalfa meal and vermicastings (earthworm castings). Our objectives are to 1) determine the influence of amendments on crop yield and total nutrient uptake, and 2) measure the amount and timing of nutrients supplied by the amendments.

The amendments were applied to an infertile sandy soil at three rates equivalent to 200, 400 and 800 kg ha-1 of total nitrogen (i.e. we measured the total nitrogen concentration of each amendment and standardized the application rates to provide the same total nitrogen for each treatment). The worm feedstock used to produce the vermicastings included sheep manure compost mixed with peat moss and cardboard. We grew lettuce and orchard grass in a greenhouse to measure plant yield and nutrient uptake.

The relationship between the quantity, form, and timing of N release was quite different among the amendments. In the poultry manure compost and feather meal, the mineral nitrogen was initially in the form of ammonium, which can cause toxicity problems for seedlings; the ammonium was converted to nitrate in a matter of 3-4 weeks. Most of the mineral nitrogen was in the form of nitrate in the vermicastings treatments. Initially, a mix of ammonium and nitrate was measured in the alfalfa meal, but nitrate was prevalent after 4 weeks.

Total nitrogen content was highest in the feather meal (13%) followed by poultry manure compost (6%), alfalfa meal (3%) and vermicastings (1.5%). As a result of these differences in concentration, amendment application rates varied considerably. For example, roughly 10x the weight of vermicastings was needed to provide the same level of N as feather meal. For each amendment we estimated what proportion of the total nitrogen would be available for plant uptake. As a % of total nitrogen applied, available nitrogen was 60% for feather meal, 52% for poultry manure compost, 29% for alfalfa meal and 12% for vermicastings. The low supply of nitrogen from the vermicastings is not surprising considering it consisted of a mature compost, peat moss and cardboard. These results are expected to be quite typical for feather meal and alfalfa meal. For vermicastings and poultry manure compost, however, the nutrient supplying potential can vary considerably depending on what materials are included in the mix for the compost and for the worm feedstock.

We found that doubling application rates did not double the amount of nitrogen or other nutrients that the amendments provided. High rates of organic amendments can put the nutrients in the soil out-of-balance. For example, the microbes in the soil need a certain amount of phosphorus in order to decompose organic material. If the organic material does not provide enough phosphorus, then the microbes try to remove the phosphorus from the soil. If the phosphorus is not easily available in the soil, then decomposition of the amendment will be slowed down, and the amount of phosphorus available to the plant may even be temporarily lowered. A supplement of other nutrients may be required when applying these amendments.

The lettuce responded well to the vermicastings treatment, however, seedlings were either killed or severely suppressed in the poultry manure and feather meal treatments. This is most likely related to toxicity issues resulting from decomposition. With the orchard grass, the poultry manure compost produced the highest yields followed by feather meal, alfalfa meal and vermicastings. N deficiency symptoms became evident in the vermicastings treatment as time progressed.

Economic analysis was based on the value of nitrogen in each treatment. The poultry manure compost was the least expensive, followed by feather meal, alfalfa meal and vermicastings. The cost of the vermicastings was very high. With its high peat moss content, however, the vermicastings provided many other benefits and potential uses as a potting medium.

This research was lead by Andy Hammermeister of the Organic Agriculture Centre of Canada with the valuable support of Tess Astatkie, Ekaterina Jeliazkova, Phil Warman and Ralph Martin at the Nova Scotia Agricultural College. The research was funded by the NSDAF Technology Development Program with support from Jolly Farmer, the Atlantic Canadian Organic Regional Network, Brian Ives, Archibald Farms (now Maritime Pride Poultry) and Western Ag Innovations.


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